1. Field of the Invention
The present invention relates to a transmitter circuit for reducing crosstalk noise of a receiver, that is generated in the case of transmitting a number of high speed signals in parallel, and more particularly, to a transmitter circuit which compensates for influence of crosstalk noise in a pre-emphasis scheme by estimating far-end crosstalk noise of a receiver generated due to electromagnetic coupling of other adjoining transmission lines when transmitting a number of high speed signals in parallel.
2. Description of the Related Art
In the case of transmitting a number of high speed signals through microstrip transmission lines on a printed circuit board, etc., timing jitter is induced under influence of crosstalk noise generated due to a difference between a capacitive coupling coefficient and an inductive coupling coefficient.
Crosstalk noise indicates a phenomenon by electromagnetic coupling of respective signal lines, and means noise that is generated as signals on different transmission lines influence adjoining transmission lines by coupling such as electrostatic coupling, electromagnetic coupling, and so forth. That is to say, when a number of transmission lines exist in parallel and data passing through the transmission lines transit from a high level to a low level or from a low level to a high level, jitter is induced due to crosstalk, noise generated by a difference between mutual inductance and mutual capacitance.
FIG. 1a is a conceptual waveform diagram in a transmitter W and a receiver in the case where a conventional data transmission method is used in microstrip transmission lines on a printed circuit board.
In the case of strip transmission lines, since a capacitive coupling coefficient and an inductive coupling coefficient are equal to each other, crosstalk noise becomes 0. However, in the case of microstrip transmission lines which are formed on a printed circuit board, since an inductive coupling coefficient is greater than a capacitive coupling coefficient, crosstalk noise of a receiver has a negative value.
As shown in the drawing, in the case where two transmission lines exist in parallel, data DATA1 and DATA2 reach a data receiver in a state in which they are delayed by different delay amounts under influence of crosstalk noise generated depending upon whether or not the data DATA1 and DATA2 passing through transmission lines transit and to which direction the data DATA1 and DATA2 transit. The difference in delay amount can be expressed as in the following Mathematical Expression 1.
                                          T            de                    -                      T            do                          =                                                            L                s                            ⁢                              C                t                                              ⁢                      (                                                            L                  m                                                  L                  s                                            -                                                C                  m                                                  C                  t                                                      )                                              [                  Mathematical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          1                ]            
Here, Tde designates a transmission time when the transition directions of the data DATA1 and DATA2 are the same, and Tdo designates a transmission time when the data DATA1 and DATA2 transit in opposite directions. Ls designates a self-inductance, Lm a mutual inductance, Cm a mutual capacitance, and Ct the sum of a self-capacitance and a mutual capacitance.
There are three modes including an odd mode in which the transition directions of the data DATA1 and DATA2 are different, an even mode in which the transition directions of the data DATA1 and DATA2 are the same, and a static mode in which no one of the data DATA1 and DATA2 transits.
When applied signals increase with respect to time, since crosstalk noise in a receiver becomes the shape of a negative pulse, the crosstalk noise in the receiver delays signal change with respect to time in the even mode and accelerates signal change with respect to time in the odd mode. Accordingly, in the odd mode, the data DATA1 and DATA2 are transmitted by being delayed least, and in the even mode, the data DATA1 and DATA2 are transmitted by being delayed most.
FIG. 1b is a timing diagram of a conventional data transmission circuit.
In the conventional art, as shown in FIG. 1b, in a data receiver, in the case of the odd mode in which the transition directions of data are different, when data transits to a high level, since a counterpart signal transits to a low level, crosstalk noise is generated in a positive direction, whereby a receiver final signal reaches earliest. In the case of the static mode, when data transits to a high level, since a counterpart signal does not transit, crosstalk noise is not generated, whereby a receiver final signal reaches intermediately. In the case of the even mode, when data transits to a high level, since a counterpart signal also transits to a high level, crosstalk noise is generated in a negative direction, whereby a receiver final signal reaches latest. Therefore, jitter is induced in the receiver due to the crosstalk noise.
Conversely, when a capacitive coupling coefficient is greater than an inductive coupling coefficient, data DATA1 and DATA2 reach a data receiver in a state in which they are delayed under influence of crosstalk noise generated depending upon whether or not the data DATA1 and DATA2 passing through transmission lines transit and to which direction the data DATA1 and DATA2 transit. The difference in delay amount can be expressed as in the following Mathematical Expression 2.
                                          T            do                    -                      T            de                          =                                                            L                s                            ⁢                              C                t                                              ⁢                      (                                                            C                  m                                                  C                  t                                            -                                                L                  m                                                  L                  s                                                      )                                              [                  Mathematical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          2                ]            
The Mathematical Expression 2 represents a case in which the signs of the Mathematical Expression 1 are reversed. In other words, when a capacitive coupling coefficient is greater than an inductive coupling coefficient, in the odd mode, the data DATA1 and DATA2 are transmitted by being delayed most, and in the even mode, the data DATA1 and DATA2 are transmitted by being delayed least.
As a consequence, in the conventional art, when an inductive coupling coefficient is not only greater but also less than a capacitive coupling coefficient, crosstalk noise is generated between the data DATA1 and DATA2 depending upon whether or not the data DATA1 and DATA2 transit and to which direction the data DATA1 and DATA2 transit, whereby the data DATA1 and DATA2 reach the data receiver with a difference in delay amount. Due to this fact, a problem is caused in that jitter is induced in the data receiver, and resultantly, as a time margin of the data DATA1 and DATA2 decreases, high speed signal transmission is impeded.